feat: set a master locale when creating a repository

[angeloashmore]

Resolves: #207

Description

prismic init and prismic repo create create a repository with no locale, so you can't add content until you manually run prismic locale add en-us --master (and the Migration API fails without one).

This adds a --lang option (defaulting to en-us) to both commands that sets the new repository's master locale on creation, making a freshly created repository usable out of the box.

Checklist

• A comprehensive Linear ticket, providing sufficient context and details to facilitate the review of the PR, is linked to the PR.
• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.

Preview

How to QA ¹

Run prismic repo create (or prismic repo create --lang fr-fr) and confirm the new repository has a master locale set. The same applies to prismic init when it creates a repository.

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Note

Medium Risk
Touches repository provisioning and locale API calls; a failed upsert leaves a repo without a master locale, though errors are surfaced explicitly.

Overview
New repositories were created without a master locale, blocking content and Migration API use until someone ran prismic locale add manually.

prismic repo create and prismic init (when it creates a repo) now accept --lang / -l, default en-us. Shared createRepo calls upsertLocale with isMaster: true right after createRepository, with clear errors if locale setup fails.

An integration test asserts --lang fr-fr sets the master locale on the new repo.

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[jomifepe]

LGTM 💯